Heat Stress and Aerobic Exercise

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Heat Stress and Aerobic Exercise

• Sweat Rate and Blood Flow

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Heat Production
Heat

• 42 38

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Heat Production

• A naked human can maintain a constant body
  temperature in 54-60 C
• Exercise in the heat results in heat gain
• Stimulants
• Sweating is the primary means of heat loss

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Heat Loss

• Increase sweat rate
• Increase skin blood flow

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Sweat Response

• During exercise, heat production in active
  muscles can be 100 times greater that of inactive
  muscles
• During moderate exercise, if a person could not
  lose heat, body temperature would increase
  approximately 0.2 C per minute
• This would cause thermal injury within 15-20
  minutes.

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Sweat Response

• Sweating begins seconds after exercise begins
• Sweating reaches an steady state after 30 minutes
• During exercise, sweat rate is related more to
  intensity of exercise than environmental
  temperature

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Sweat Response

• Sweat glands are stimulated by the hypothalmus
• Sweat is formed in the sweat gland
• In the glands, Na and Cl are reabsorbed into the
  body (surrounding tissues and blood)
• Sweat is hypotonic

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Sweat Response
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Sweat Response

• Fluid loss as little as 1 of body mass can lead
  to an increase in core temperature
• For each liter of sweat-loss dehydration,
  exercise HR increases by 8 bpm while Q decreases
  1 liter/min
• Fluid loss of 4-5 body weight shows a definite
  impairment in performance

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Sweat Response

• Peak sweat rate 1-2 liters per hour during
  exercise
• 1 liter 2.2 pounds
• For every pound lost through dehydration
• Q decreases by 1 liter per min
• Body temperature increases by 0.3 C (0.54 F)
• Fluid ingestion is required to maintain such high
  sweat rates

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Sweat Response

• Goal of fluid replacement is to maintain plasma
  volume so that circulation and sweating wont be
  impeded
• Ad libitum water intake replaces only half of the
  water lost.
• Thirst is not a good indicator of level of
  dehydration
• Recommendation
• 6-10 ounces every 10-20 minutes
• Water versus sport drinks

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Blood Flow

• Goals
• 1. dissipate heat with increased skin blood flow
• 2. maintain blood flow to active muscles via
  cardiac output
• With extreme heat stress, 15-25 of the cardiac
  output (Q) goes to the skin
• Why could this be a problem during exercise?
• Reduced blood flow to muscles
• Redistribution of blood flow away from the
  muscles
• Reduced blood volume

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Blood Flow

• The greater the heat stress during exercise, the
  more vasodilation of muscle and subcutaneous
  blood vessels
• Also, vasoconstriction of organ blood vessels
• Possible renal and liver complications

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Blood Flow
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Blood Flow
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Blood Flow

• During exercise, plasma volume decreases
• Increase blood pressure
• Sweating
• What will happen to preload?
• Stroke volume?
• How is cardiac output maintained?
• Higher heart rate

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Blood Flow
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Blood Flow

• With dehydration, an increased HR is unable to
  maintain Q at higher intensities
• Therefore, Q will be lower.

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Blood Flow

• What affect would dehydration have on blood
  pressure?
• How does the body prevent this?
• Sends less blood to the periphery to maintain
  pressure
• Also, near maximal effort, blood vessels
  constrict to help maintain blood pressure

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Blood Flow

• What affect does this vasoconstriction have on
  body temperature?
• This decreases heat loss and core temperature
  increases
• Therefore, the greater the intensity level
• greater heat production
• less heat loss

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Blood Flow Summary

• Heat loss mechanisms compete with active muscles
  for blood flow
• Skin blood flow
• Increase sweat rate
• Decrease plasma volume
• Although Q may remain constant, SV may decrease
  forcing HR to increase
• Vasoconstriction to maintain blood pressure
• Decease blood flow to skin, further increase in
  core temperature

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Oxygen Supply
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Acclimatization

• 7-10 days
• 2-4 hours per day
• Exercise sessions should begin with light
  exercise for 15-20 minutes and gradually increase
• Hormonal control attempts to conserve water
• Resting plasma volumes can increase 10-20 with
  repeated days of exercise in the heat

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Acclimatization

• Improved blood flow
• Better distribution
• Lower sweat threshold
• Better sweat distribution
• Increased sweat output
• Lower Na in sweat
• Lower submax temp and HR
• Less reliance of carbohyrdates

• Better heat loss
• Better heat loss and BP
• Earlier cooling
• Better cooling
• Better cooling
• Preserves Na
• More Q to muscles
• Carbohydrate sparing affect

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TR
HR
Sweat loss
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Acclimatization

• Exercise training in neutral temperatures causes
  adaptations similar to training in the heat.
• Such changes typically take 8 to 12 weeks
• Therefore, aerobicly trained individuals adapt
  better to exercise in the heat than untrained
  individuals
• Changes are earlier and greater sweating and
  increase in plasma volume
• Complete acclimatization is not possible without
  training in the heat

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Questions?